Tactics to avoid economic losses to common scab
By Dave Antrobus, Technical Services Lead - Syngenta
Fundamental to avoiding significant losses to common potato scab are background knowledge about the paddock you’re planting into and an understanding of how a combination of different control measures will work best for you.
For growers, there is a lot of information on common scab. Agriculture Victoria has a good technote (AG0313) and more valuable material is available at the Cornell University website. I’ll summarise their main points here.
Common scab is caused by Streptomyces scabies. This disease overwinters in the soil and can also be seed-borne. It can survive for a very long time in alkaline soils and becomes increasingly scarce in acidic soils.
Do you know your soil pH?
Information you should know is the pH of your soils. As a rule, within soil pH range of 5 to 8, the incidence and severity of potato scab increases with increasing alkalinity. Scab usually remains at a satisfactorily level where soils are pH 5.0 to 5.2. At neutral pH of 7.0 and above, this disease can be severe.
For growers considering liming, it may be best to hold off until after the potato crop is grown and harvested as applications in the lead up to planting can greatly increase the risk of common scab infection.
Fertilisers that impact on soil pH can also impact on this disease. A lot of people think that animal manures lower soil pH in the same way that commercial nitrogen fertilisers do. Many animal fertilisers however, for example, those sourced from chicken egg farms, can raise soil pH substantially. The main reason is due to the lime like materials such as calcium and magnesium in the manure. Similarly, wood ash can make the soil more alkaline.
The lowering of soil pH with applications of sulphur has proven useful in reducing scab in some soils of high pH. Use of acid-producing fertilisers e.g. ammonium sulphate as a source of nitrogen can help.
So, over the longer term, try to maintain soil pH levels between 5.0 and 5.2 by using acid-producing fertilizers such as ammonium sulphate. Avoid or limit the use of such alkaline-producing amendments as lime and manure.
What other techniques can you use?
Long rotations of five years or more between potato crops, preferably with legumes (but excluding beets, carrots, parsnips and fleshy-rooted crucifers), are useful in reducing the severity of potato scab.
If your paddocks have a history of scab, plant resistant varieties, obviously avoid susceptible ones and only sow clean certified seed.
MAXIM® 100FS is a seed treatment that will reduce the effects of common scab. It can be applied to the seed prior to storage, or at planting. In addition to providing suppression of seed-borne common scab, Maxim 100FS is relatively broad-spectrum and can also control black dot, fusarium dry rot, black scurf (Rhizoctonia spp.) and silver scurf.
Syngenta is also expecting to bring a new seed treatment option to the market in late 2018, namely VIBRANCE® PREMIUM, which is currently under review by the APVMA. It too will assist in reducing levels of common scab as well as many other diseases.
Dry soil when tubers start to form, and for the five weeks after that, will increase the chances of common scab occurring. For this reason, growers should water regularly over this time and maintain soil moisture at field capacity. Pay close attention to irrigation on coarse-textured soils, because of their reduced moisture-holding capacity. Gravelly or sandy areas tend to dry out more rapidly and are often sites of heavy scab infection.
As with all disease control strategies, the combination of as many different methods as possible will ensure the best results.